The present invention relates to a subscriber line interface circuit and, in particular, to a subscriber line interface circuit having an interfacing capability between analog and digital circuits.
Such a subscriber line interface circuit has been heretofore connected between an analog telephone terminal and a digital switching system and comprises an interface circuit and an encoding and decoding circuit.
FIG. 1 is a block diagram of a subscriber line interface circuit showing such a prior art.
The prior art subscriber line interface circuit comprises an interface circuit 1A and an encoding and decoding 2A (hereinafter referred to as CODEC) having a single power source as shown in FIG. 1. In these circuits, an analog output A.sub.O of the CODEC 2A is connected with an input terminal RX of four-wires 4W (hereinafter referred to as four-wire side input terminal) of the interface circuit 1A via a capacitor C2. A resistor R4 is connected between the RX terminal and the ground (GND). An analog input A.sub.I of the CODEC 2A is connected with an output terminal TX (hereinafter referred to as four-wire side output terminal) via a capacitor C1. A resistor R3 is connected between the terminal A.sub.I and a reference voltage output terminal (V.sub.RO). These capacitors C1 and C2 serve to eliminate a d.c. bias voltage which is a reference voltage imposed upon the analog input output terminals A.sub.I and A.sub.O for passing only an a.c. signal.
When the analog output A.sub.O of the CODEC is directly connected with the terminal RX of the interface circuit in the afore-mentioned prior art subscriber line interface circuit having the encoding and decoding circuit with a single power source and the interface circuit, power supply to the two-wire side is influenced by the d.c. bias voltage at the output A.sub.O of the CODEC. Accordingly, a d.c. eliminating capacitor C2 is used to eliminate an influence of the direct current. However, since the signal which passes through the capacitor is analog, the capacitor requires a relatively high capacity. The analog input A.sub.I of the CODEC surely requires the capacitor C1 since the voltage on the interface circuit side is 30 to 40 volts which are larger than that of the terminal A.sub.I.
The prior art subscriber line interface circuit has a disadvantage that the frequency characteristics becomes worse since the interface circuit is usually coupled with the CODEC via capacitors in such a manner. In the prior art circuit, a high capacity capacitor cannot be built-in an LSI and should be externally added. Therefore, there is a disadvantage that the line circuit on which the capacitors are mounted is cumbersome and expensive.